The invention relates to a filter chain for letting pass a frequency band, comprising a number of filter units made of quartz or other high-quality piezoelectric material, which filter units: vibrate according to the thickness shear mode, are coupled electrically one to another, and are each provided with two acoustically coupled resonators.
The developments in the telecommunication technique tend to increasingly higher carrier current frequencies, so that, for the sake of filtering a narrow frequency band out of these high frequencies, filters with a very small relative bandwidth have become needed.
With a filter consisting of two acoustically coupled resonators, a narrow pass band can, in consequence of the high Q factor, only be realized by means of a small acoustic coupling. The drawbacks of such filter units, however, are a high pass-band attenuation and a high sensitivity for signal levels. Also, the manufacture and the possibility of reproduction of such filter units are difficult and expensive. Moreover, it may be a drawback that the bandwidth of such filters cannot, or only difficulty, be adjusted from the outside after being manufactured.
The problem posed can be solved by starting from a different approach, by which the filter chain comprises one or more filter sets, each having two electrically coupled filter units with at least one equal resonant frequency so that the acoustic coupling coefficient within one filter set is very high with respect to the electric coupling coefficient. This embodiment causes four tuned circuits with three couplings, two of them having a high coupling coefficient and one a low adjustable coupling coefficient. Two of the four resonant frequencies fall within the desired pass band and the other two far outside this pass band. Within a filter set the symmetric resonant frequency of one of the filter units is preferably equal or almost equal to the anti-symmetric resonant frequency of the other filter unit. The acoustic coupling coefficients have to be very high, but not necessarily equal to one another.
However, with the above-mentioned narrow-band filter, it is difficult to maintain the frequency stability. According to known methods, a harmonic filter is made use of in order to obtain a high frequency stability and a high circuit quality Q.
The results thus obtained fully meet the demands made with regard to the frequency stability. However, another drawback is the decrease of the distance between the resonant frequency and the anti-resonant frequency, in consequence of which undesirable frequencies will come too near to the pass-band area.
To obtain a larger distance between the resonant frequency and the antiresonant frequency, the acoustic coupling coefficient must be increased considerably. For this purpose it has been proposed to make use of relatively thin electrodes and to make the distance between the electrodes very small. Although these measures indeed produce some effect, the acoustic coupling on the one hand cannot be made so strong that there is considerable improvement of the distance between the resonant frequency and the anti-resonant frequency, whereas on the other hand there is very little possibility of reproducing of such filter units.